N,N′-Carbonyldiimidazole (hereinafter abbreviated to CDI) is a useful compound used in the fields of synthesis of pharmaceutical agents, synthesis of agricultural chemicals, peptide synthesis, and the like, e.g., intermolecular condensation reactions, intramolecular condensation reactions for synthesizing N-carboxylic anhydrides, production of activated esters, and the like. The most general synthetic method of CDI is a method of reacting imidazole with phosgene in an inert solvent, which is contrived by H. A. Staab. In this method, as shown in the below-shown chemical formula, a half of the imidazole is utilized as a capture agent for hydrogen chloride generated during the reaction and is separated and removed from CDI after completion of the reaction.

For the purpose of utilizing a whole amount of imidazole, there is disclosed a method of using an organic base as a capture agent for hydrochloric acid generated (e.g., Patent Documents 1 and 2). Moreover, a method of silylating imidazole with a silylating agent such as trimethylsilyl chloride has been performed so that hydrochloric acid is not generated. However, in the case of using an organic base, since the organic base is not completely inactive to phosgene, impurities tend to be produced and remain in the product CDI, the presence of the impurities being a factor of purity decrease. In the case of silylating imidazole, a silylating agent can be circulated and utilized with suppressing generation of hydrochloric acid but it is necessary to make associating capital investment for special handling of the silylating agent which is not generally stable. These methods mentioned above are difficult to utilize as industrial processes for obtaining highly pure CDI at a low cost.
Therefore, there has been used a method of neutralizing the imidazole hydrochloride yielded as a by-product with an inorganic base such as sodium hydroxide or an aqueous solution thereof, recovering the imidazole, and returning it as a starting material for CDI production (e.g., Patent Documents 3 and 4). However, also in the method, when the imidazole is recovered using water as a solvent, labor and cost are required for extraction with an organic solvent immiscible with water since imidazole is soluble in water. Furthermore, even when the imidazole is recovered using an organic solvent, since CDI is severely highly hydrolyzable, it is necessary to incorporate a step of removing water generated during the neutralization reaction.
Patent Document 1: WO98/31672 pamphlet
Patent Document 2: WO00/02863 pamphlet
Patent Document 3: WO00/14072 pamphlet
Patent Document 4: WO00/06551 pamphlet